The present invention relates to methods for making tennis balls and the like, and particularly to a method for bonding the tennis core and fabric cover pieces to one another through a single bonding step.
The art of manufacturing tennis balls has been well developed heretofore by such companies as Dunlop Rubber Company, Ltd., of London, England as shown by its U.S. Pat. Nos. 4,151,029 and 4,248,658 and Great Britain Pat. No. 436,100. The elastomeric core of a tennis ball is usually made of a vulcanized rubber molded into a pair of hemispherical rubber cups which are then adhered together by a heat curable cement. The core is heat treated to bond the hemispherical rubber cups to one another before the fabric cover is applied.
The manufacture fabric covers for tennis balls generally include the cutting of a sheet of fabric into "dumb-bell" shapes, a pair of such fabric dumbbells being applied about the core to envelop it. The wide seam visible between the fabric dumbbells on a completed ball is provided by a suitable adhesive, usually a white rubber solution, which is applied to the edges of the fabric before the fabric is assembled to the core.
Fabric to core adhesives or cements are also well known in the art. Once the core has been completed, the core is dipped in the fabric to core adhesive or cement and then allowed to dry until the adhesive becomes tacky. The fabric dumbbell shapes are then applied manually or by automated equipment about the cores with the white cement between the fabric edges. This assembly is then subjected to a heat curing step whereby the fabric is bonded to the core and the edge seams are cured as well.
Since tennis balls are internally pressurized to provide a desired bounce characteristic, the core halves are assembled in a pressurized environment so that a suitable internal pressure, on the order to seventeen (13) pounds per square inch, is provided within the core during its initial manufacture. When the core is subjected to the adhesive heat curing step, the mass of air within the core tends to expand the core against the surrounding heat applying mold. It was thus necessary to allow the mold and core to cool sufficiently after a core curing step so that the core will not blow part under the elevated internal pressure caused by the heated mass of air contained therein. The core is also subjected to a second heating step when the fabric is bonded to the core.
It has come to my attention that it would be time saving and more economical to be able to assemble a tennis ball core and fabric cover in a completed assembly for a single cement curing step. As will be discussed more fully hereinafter, in order to accomplish a single step bonding method as disclosed hereinafter, the within method utilizes a core having a one-way air valve or check valve provided in the core as disclosed in my prior U.S. Pat. Nos. 4,240,630 and 4,327,912. As discussed in my prior patents the check valve formed integrally of the core provides for the introduction of air after completion of the manufacture of the tennis ball so that balls that lose their pressure, can be repressured.
It has also come to my attention that it would be more economical and easier, from a ball manufacturing standpoint, to be able to utilize lightweight molds and less expensive equipment in curing the adhesive to bond the ball core halves and fabric together.